Updated on January 8th to set things right from chapter 8 to chapter 1 (prologue)
Early Note: It's safe to say that I can gives you any information that you need to know according to my chapters advancement.
Between Ranks and Level :
Adventurer ranks and Falna level is respectively different. Consider in ranks, it is needed to prove yourself worthy for it, but in Falna level, it's needed some kind of experience points to level up. It's also possible for someone not joined a Familia but he's an adventurer.
Example : Someone has achieved Falna level 5 but doesn't have any ranks. When he became an adventurer, he must started from D-rank. Whether he wants to rank up or not, it depends on himself.
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Dungeons :
Not just with the Tower of Babel in Orario, I made a several places that holds a dungeon. It may had a different look from Babel but you'll find it later in my story advancement.
Guild :
It's like a guild in any other fantasy literature(s). It is built to accommodate an adventurer to take a quest. It's also coordinate things with other Gods/Goddesses.